The ventilating system of a laboratory building (or of a laboratory subdivision of a building) is distinctive; it contrasts with the ventilating system of a general purpose building. In the latter, it is customary to recirculate most of the air within a building, discharging a small percentage of it from the building and replacing that discharged with fresh air from outside the building. In contrast, the air taken into a laboratory building is comfort-conditioned and supplied both to non-laboratory areas and to laboratory rooms and the total volume of that comfort-conditioned air delivered to laboratory rooms is discharged from the building. Particularly because the comfort-conditioned air is not recirculated, any air that is needlessly discharged as exhaust from the fume hoods of laboratory rooms constitutes substantial waste. Air supplied to laboratory rooms is exhausted from the room through the fume hoods.
A fume hood is open at the front to provide access to the experimental equipment and material contained in the hood. A normally closed sash shuts the fume hood's access opening; the sash is opened adjustably as needed for access to the experimental set-up. Exhaust air, or "exhaust", is drawn from the room into the fume hood and then into an exhaust duct, for assurance against fumes entering the laboratory room. The exhaust flow of a single fume hood may be induced by a dedicated variable-capacity fan. However, among many fume hoods that discharge exhaust into a common duct, each fume hood has its own adjustable air valve or damper, commonly called a "variable air volume box" or "VAV box". The exhaust "volume" or volumetric flow rate is measured in cubic feet per minute, or "CFM", and exhaust flow is induced by a negative pressure gradient in the exhaust duct, with pressure becoming more negative in the direction of exhaust flow toward the fan.
A fume hood characteristically has some form of bypass passage for allowing a minimum flow of air through the fume hood while its sash is closed; the purpose of this is to continuously ventilate the cavity in the hood to avoid a build-up of a high concentration of fumes within the hood. Consequently, the VAV box is maintained open sufficiently to sustain a minimum flow of air into the hood through the bypass passage.
The volumetric flow rate of air into a hood should be great enough to develop a safe "capture velocity" at all points across the plane of the hood sash opening to ensure a sufficient velocity to assure entrainment of fumes into the hood and thus prevent escape of fumes into the laboratory room. The average velocity of air entering all the unit areas of the sash opening is called the "average face velocity". The average face velocity should be high enough to develop the required capture velocity as well a to insure sufficient face velocity at any local point in the plane of the hood at any hood sash opening.
For economical use of the air supplied to a laboratory room, the dedicated exhaust fan or the VAV box is adjusted in coordination with the sash opening. The two basic types of control mechanisms for achieving this goal are known. According to conventional wisdom the volumetric rate of air flow into the hood sash opening should be varied linearly with changed sash openings for both types of control of the exhaust flow rate.
One form of exhaust flow control for a fume hood depends on an air velocity sensor in a passage from the space in front of the fume hood to the space inside the fume hood cavity, called a "face velocity sensor". Commonly, that sensor is an electronically heated sensor that is cooled variably as a function of the air velocity across it through the passage. The sensor is part of a control circuit designed to maintain constant air velocity past the sensor. As the sash opening changes, the control circuit adjusts the volume flow rate. This form of control over the volumetric flow rate of air through the fume hood is primarily used for fume hoods in which the sashes are encased in a panel with vertical movement of the encasement and with work panels that slide in the encasement horizontally (i.e. "combination sash" hoods) or in hoods where the base panels can only slide horizontally in a track (i.e."horizontal sliding sash" hoods).
In another form of exhaust flow control for a fume hood, a sash position sensor is used to control the volumetric exhaust flow rate. For example, the sensor may be a potentiometer or a 3-15 psig control valve coupled by a cable to the sash or geared to turn with displacement of the hood sash. Commonly, this form of control is used for fume hoods in which a single sash panel is adjusted vertically.
The control of the volumetric flow rate of exhaust discharged by a fume hood or fume hoods of a laboratory room reflects on the supply of air into the laboratory room. This is so, in part, because a laboratory room is supplied with comfort-conditioned air from a supply duct at a rate controlled by a VAV box which, in turn, responds to a signal representing all of the laboratory room's exhaust flows. The flow rate from the supply duct into the laboratory room is normally controlled to be slightly less than (or in select instances greater than) the total exhaust flow rate, to establish either a slightly negative laboratory room pressure (for guarding against escape of fumes from the laboratory room) or a slightly positive pressure (for guarding against airborne particles entering a "clean room".) In the more common situation where infiltration into a room is desired, the difference between the controlled supply volume of air into a laboratory room and the larger total of all exhaust flows out of the laboratory room is made up by a supplemental flow of air into the laboratory room from a corridor or other non-laboratory area adjoining the laboratory room. The difference between the controlled room supply and exhaust is the infiltration air and it moves through constrictions such as the gap between a laboratory-room door and its sill, to sustain the laboratory room's negative pressure difference relative to the non-laboratory area.
The exhaust flow from a laboratory room may be only the exhausts of the fume hoods of that laboratory room. However, the laboratory room exhaust may include air that is drawn out of the laboratory room through an air valve that responds to a room thermostat. In this way, comfort-conditioned air can be supplied to the laboratory room even when the combined fume-hood exhaust flow is not sufficient during periods long enough to maintain the laboratory room at a comfort level.
Supply of air to the laboratory rooms and other rooms and non-laboratory areas entails certain recognized constraints, notably control of pressurization of the building. Efforts have been devoted to maintaining the air pressure inside a building neutral relative to the ambient atmospheric pressure (i.e. avoiding infiltration into the building or exfiltration from the building). If the pressure inside the building deviates significantly from the sustained pressure outside the building, comfort-conditioned air may be expelled, a costly waste; or external air that is not comfort-conditioned may be drawn into the building. Moreover, a seemingly small inside-to-outside pressure difference can develop a large and potentially destructive force acting on a large wall or window area. Static pressure sensors have been tried for maintaining neutral pressurization, but satisfactory low-cost, high-sensitivity sensors for such low pressure levels are, at least, very expensive and very difficult to find. Additionally, any such pressure sensor inside a building is vulnerable to the effects of winds at the windward and leeward sides of the building. Winds tend to cause spurious local pressure changes inside the building, affecting such highly sensitive static pressure sensors.